Physicians often prescribe medical oxygen to certain patients such as those with chronic obstructive lung disease, restrictive lung disease, cystic fibrosis, bronchiectasis, or lung cancer. The medical oxygen is typically delivered to the patient via a supply tube called a nasal cannula, which is connected to a pressurized oxygen source. A gas regulator is typically employed to reduce the source pressure and meter the flow rate of the oxygen to the patient.
If the patient is bedridden in an institutional setting, such as a hospital or nursing home, the oxygen is frequently delivered to a wall port and supplied by a large bank of oxygen storage tanks. In such situations, the patient is often provided with a continuous flow of oxygen.
While oxygen delivery through a typical continuous flow nasal oxygen cannula is effective at oxygenating patients, it is wasteful. In particular, oxygen continues to be delivered during exhalation, which prevents the oxygen from reaching the patient's alveoli. In addition, oxygen delivered during late inhalation does not participate in alveolar gas exchange and therefore does not substantially oxygenate the patient.
The wasteful periods of oxygen delivery can be described with reference to the volume/time breathing cycle. The typical patient spends about ⅔ of the ventilatory cycle in exhalation. Oxygen delivered during that time will not flow toward the alveoli and therefore will be wasted.
During inhalation, about the last ⅓ of that volume fills the dead space (airways leading to the alveoli, but not into the alveoli). Because inhalation begins with a rapid upstroke but slows at the end, dead space inhalation occupies about the last ½ of the inhalation time. Oxygen delivered during this part of the ventilatory cycle is also substantially wasted.
All told, roughly the last ⅚ of oxygen delivery to the ventilatory time cycle is substantially wasted. Ideally, all of oxygen delivery should be targeted to the first ⅙ of the ventilatory cycle, which represents about the first ½ of the inhalation cycle. This is called “early inhalation”, during which oxygen can flow into the alveoli and become available to the alveolar capillaries and to the blood circulation so that it can be delivered to the cells to support and enable metabolism.
Many patients are mobile and rely on oxygen stored in a portable oxygen cylinder. Because so much oxygen is wasted by continuous oxygen flow delivery, the limited supply of stored oxygen can be quickly depleted, especially for the smallest and most mobile cylinders, thus severely limiting the amount of time the patient can be active. Also, those patients require a higher flow setting due to the fact that patients who are exerting or exercising consume more energy and, consequently, more oxygen. To prolong the portable oxygen supply, numerous techniques have been devised to conserve the oxygen and optimize the oxygen delivery efficiency.
One technique to maximize oxygen delivery to the alveoli is to deliver oxygen through a transtracheal catheter through a surgical incision in the patient's neck. While this allows the oxygen to be delivered more directly to the alveoli, this technique is a surgical procedure that can be uncomfortable to the patient and introduces the risk of complications, including infections. Moreover, the transtracheal catheter requires a program of care.
Another technique employs pulse devices to deliver oxygen either periodically or on demand. The typical demand conserver senses the beginning of an inhalation and delivers a short pulse of oxygen in response. While the goal of the on-demand conservers is to optimize oxygen delivery, they vary in effectiveness. The conservers can be electronically or pneumatically controlled and often share a housing with the gas regulator.
Yet another technique employs a reservoir cannula. These devices typically use a membrane to form a sealed reservoir to store oxygen during exhalation for delivery during the next inhalation. Examples of typical reservoir cannula systems are described in U.S. Pat. No. 4,535,767 to Tiep et al., U.S. Pat. No. 4,572,177 to Tiep et al., and U.S. Pat. No. 7,328,703 to Tiep, the teachings of which are incorporated herein by reference in their entirety. Although these devices have been utilized for high flow delivery, the benefit of a storage chamber is defeated by the high flow (i.e. greater than 8 L/min) as the membrane is typically pushed to the open position and cannot cycle.